The electrical energy production is at the moment going through major changes. The pollution and greenhouse gas emissions of the energy sector have gained increasing attention. At the same time as the electrical energy production is moving towards renewable energy based energy production, the electrical grid is also facing new challenges. Previously, the power plants connected to the electrical grid were very large such as nuclear power plants, large coal-based power plants, etc. This centralized electrical power generation, of course, causes losses in the electrical grid because the energy must be transferred over long distances.
Distributed power generation is closer to the consumption and thus smaller losses occur in the electrical grid due to shorter distances over which the energy is being transferred compared to centralized power generation. In contrast to the centralized power generation plants which typically operate at their rated power, the distributed power generation plants have to be able to constantly adjust their operation and output power based on load demand. This is especially important if the electrical grid seizes to feed or receive power such as in case of islanding conditions during which the power generation and power consumption must be equal typically in a rather small area wherein the electrical grid is typically weak. In these cases, the operation is entirely relying on the control and operation of a single power generation unit or few power generation units. It is, therefore, of utmost importance to have power plants which can run at high efficiency also at part-load conditions and can adjust their output rapidly.
One major reason why the amount of distributed energy production has not increased more is the higher price of the energy produced by these systems compared to, e.g., price of the electricity from the grid. This is typically due to a lower electrical efficiency compared to large power plants. Gas engine and gas turbine plants, which are more and more being used in the electrical energy production, are good examples of power plants that can be utilized in distributed power generation.
Gas turbine plants are typically designed to operate at 100 percent of the nominal load, i.e. the design point. Nowadays, the electrical production efficiencies of commercial gas turbines at their design points are at the most around 40 percent, especially, in the plants with electrical power rating less than 20 megawatts. The electrical efficiency which itself is not very high, quickly decreases if the gas turbine is being operated at part-load conditions, i.e., at load conditions less than 100 percent of the nominal load.
A typical gas turbine power plant comprises a compressor, a combustor, a turbine and an electrical generator. The compressor and the turbine are mounted on the same shaft and form a single spool. The generator is also mounted on the shaft. Some prior art, however, describes solutions with gas turbines having two spools. Two-spool arrangement offers potentially better efficiency than a single-spool system because more power can be produced with the same turbine inlet temperature compared to a single-spool system.
Typically the two spools of the gas turbines are different in a way that there is a high pressure spool and a low pressure spool. Low pressure spool is typically connected to the main electrical generator while the high pressure spool is operating as a gas compressing spool. These kinds of two-spool designs are available from approximately 15 megawatts and upwards. In these two-spool designs, the designing of turbine impellers may become more challenging due to their size and effect on the rotor dynamics. The high ratio at which the speed of the spools changes relative to each other further complicates the design and control of the system.
In some attempted solutions of the gas turbine plants, two spools have been utilized wherein both of the spools have electrical generators coupled to their shafts. In these solutions, the power taken out of the gas turbine plant has been taken mainly from a single electrical generator, that is a main generator, and the other generator has been working as an auxiliary motor/generator, typically having lower power rating than the main generator and having a rotational speed at different speed range than the main generator. There are also solutions in which both of the electrical generators have been used primarily for controlling the operation of the gas turbine plant, thus both of the generators being auxiliary motors/generators, while the power taken out of the gas turbine plant is mainly taken from an additional free turbine spool to which an additional generator, operating in these cases as the main generator, is connected to.
The high speed of the spools, in general, may introduce difficulties and wears off bearings quickly and causes high losses due to friction. Especially in distributed power generation, the output power of the power generation plant must be able to be changed rapidly. In typical gas turbines, the control may not be sufficiently rapid to react on all load changes or changes in outputs of other power generation plants. In gas turbines, control capabilities can be affected by designing, e.g., the dynamical properties related to blades and impellers as well as the structure and control of the generators.